A liquid crystal display for displaying a still picture and a moving picture has conventionally been employed in a personal computer, a television receiver, a portable telephone, a portable information terminal and the like.
FIG. 17 is a circuit diagram showing a main part of such a liquid crystal display. In FIG. 17, this liquid crystal display includes a liquid crystal cell 70, a scan line 71, a common potential line 72, a data signal line 73 and a liquid crystal drive circuit 74, wherein liquid crystal drive circuit 74 includes an N-type TFT (Thin Film Transistor) 75 and a capacitor 76.
N-type TFT 75 is connected between data signal line 73 and a data holding node N75 and having the gate connected to scan line 71. Capacitor 76 is connected between data holding node N75 and common potential line 72. One electrode of liquid crystal cell 70 is connected to data holding node N75 and the other electrode thereof receives a reference potential VR. Common potential line 72 is applied with a common potential VC. Scan line 71 is driven by a vertical scan circuit (not shown) and data signal line 73 is driven by a horizontal scan circuit (not shown).
When scan line 71 is set to “H” level, N-type TFT 75 is made conductive to charge data holding node N75 to a level on data signal line 73 through N-type TFT 75. A light transmittance of liquid crystal cell 70 increases to the maximum when data holding node N75 is at “H” level, for example, while the light transmittance decreases to the minimum when data holding node N75 is at “L” level. Liquid crystal cells 70 are arranged in a plurality of rows and a plurality of columns to form one liquid crystal panel, on which one picture is displayed.
In such a liquid display, an electric charge of data holding node 75 gradually leaks to gradually decrease a potential thereof and to vary a light transmittance of liquid crystal cell 70 even when N-type TFT 75 is non-conductive. Therefore, as shown in FIG. 18, a data signal is refreshed, that is to say a data signal is rewritten to data holding node 75, at prescribed intervals.
In a conventional liquid crystal display, it was required that a plurality of scan lines 71 were selected one at a time and a data signal was rewritten to data holding nodes N75 corresponding to one selected scan line 71 while one selected scan line 71 was in a selected state, therefore, a problem has arisen that control for refreshing of a data signal becomes complex.